The invention pertains to a fastening element consisting of a screw and a large-surface metallic washer to attach insulation sheeting or insulation plates, and possibly additional waterproof sheeting onto a solid substructure, whereby the screw is provided with a drill tip, a shaft with a thread that extends over at least part of its length, and a screw head. The washer is provided with a preferably central aperture in order to insert the screw shaft.
The invention also pertains to a device to screw in such a fastening element. Said device has a support surface for the washer as well as a receptacle tube to insert the end of the screw at which the screw head is located, whereby the receptacle tube is additionally constructed as a receptacle for an axially movable and rotatable tool to take hold of the screw head.
Fastening elements of the previously mentioned type are known in many different variations (for example, U.S. Pat. No. 4,781,503 or EP-A-0283184). One of the purposes of these fastening elements is securely retaining a large-surface washer against axial movement in the immediate area of the screw head or a threadless shaft section located directly beneath the screw head. This measure is intended to yield a certain degree of safety against loads, for example, from people walking on the insulation sheeting or the waterproof sheeting applied on top of said insulation sheeting during utilization of such fastening elements on roof surfaces; in other words, the screw head's penetration towards the outside through the waterproof sheeting should be prevented if such a load is applied. One known variation utilizes a plastic disk as the large-surface washer. The plastic disk is braced against a peripheral collar arranged on the screw shaft. A different variation utilizes a metallic washer which is braced against the end of the threaded section provided on the side at which the screw head is located.
Another document (U.S. Pat. No. 4,809,568) introduced a fastening element and a device designed to screw in said fastening element, whereby this known element consists of a screw and a plastic washer. The washer has an extended hub in its central area, which means in the area of an aperture provided for the insertion of the screw shaft so that the wall of the aperture adjoins the screw threads over a relatively long section. The fact that the screw [sic] may be pushed over the thread similar to a fitting and remain in this position in preassembled condition until the final mounting process is made possible by the correspondingly small dimensions of the aperture in the washer.
However, it is in practical applications not always possible to utilize plastic washers, in particular in those sections of a roof where a subsequent waterproofing is performed by means of a flaming process. It is an absolute requirement that large-surface metallic washers be utilized in such instances. In order to facilitate the preassembly of the washer and the screw, namely, at a correspondingly large distance from the screw head, it was attempted to inject a plastic part into a larger central aperture whereby the injected plastic part adjoins the screw over a certain threaded area viewed in axial direction, thus making preassembly in the form of a fitting possible. However, it was soon established that such an additional arrangement of a central plastic part in a metallic washer is connected with very high costs, thus representing a very disadvantageous factor for mass produced articles.
In order to be able to screw in such fastening elements with a practical hand tool, it must be ensured that the large-surface washer is fixed on the screw shaft at a correspondingly large distance from the screw head because an axial alignment of the screw should be obtained while screwing in the fastening element. This axial alignment is obtained by means of the screw head engaging into the tool on the one hand, and by means of guiding the screw in the aperture of the washer on the other hand, so that at least a two-point guidance exists until the tip of the screw penetrates the solid substructure.